The Outlook 2003 product key provided by Alentus does not seem to be working. Can you please advise?

If during the Outlook 2003 automated setup program you were prompted to enter the key and it was accepted without a problem, then after the setup completed, when launching Outlook 2003, you were prompted for it a second time and then told that it is not valid, it is highly likely that this is caused by having two separate versions of Office / Office Products installed at the same time. Below is a link to a Microsoft Support article that explains the situation more specifically: